1. Field of the Invention
The invention is based on a hydraulic vehicle brake system having a service brake that can be actuated by muscle force and having a device for regulating wheel slip.
2. Description of the Prior Art
Wheel-slip-regulated vehicle brake systems are understood hereinafter to mean anti-lock vehicle brake systems (ABS), vehicle brake systems with traction control (TC), or vehicle brake systems with an electronic stability program (ESP). Such vehicle brake systems have a hydraulic unit connected between a master cylinder, which is actuatable by the driver, and at least one wheel brake. This hydraulic unit is equipped with, among other elements, magnetically actuatable multi-way valves, pumps, a pump drive, and reservoirs that supply the pumps with pressure fluid. For actuating the pump drive, a drive motor is also present. Via an electronic control unit, the drive motor and the multi-way valves can be triggered for regulating the pressure in the wheel brakes as needed. Detailed information going beyond this can be found in the discussion in the brochure entitled “Fachwissen Kfz-Technik, Sicherheits-und Komfortsysteme, Fahrstabilisierungssysteme” [Automotive Technology, Safety and Comfort Systems and Stabilizing Systems], Gelbe Reihe [Yellow Series], 2004 Edition, Robert Bosch GmbH, Stuttgart, ISBN 3-7782-2026-8, in particular beginning on page 90.
The vehicle brake systems described therein are similar to one another in terms of the layout of their hydraulic circuit diagram, but they differ in their engineering effort and expense depending on the scope of their function. In practice, for instance, different structural forms of pumps and/or a greater or lesser number of differently designed multi-way valves are used in order to achieve whatever functionality is wanted. Anti-lock vehicle brake systems, for instance, make do with non-self-aspirating pumps, known as return pumps. The object of these pumps, in a braking event involving existing wheel slip, is to pump pressure fluid from an affected wheel brake back to the master cylinder in order to lower the brake pressure. Since because of the actuation of the master cylinder by the driver taking place at that time the pressure fluid in the wheel brake is already at elevated pressure, the pump itself need not perform any suction work.
Vehicle brake systems with a TC or ESP function, by comparison, must be capable of building up a brake pressure in one or more wheel brakes, regardless of any actuation of the master cylinder by the driver, so as to eliminate the wheel slip occurring upon acceleration of the vehicle or on cornering. This requires pumps that are designed to be self-aspirating. Self-aspirating pumps are capable of pumping pressure fluid even if there is no pressure difference or only a slight pressure difference at their inlet side. An exemplary embodiment of a self-aspirating pump is already known for instance from German Patent Application DE 199 28 913 A1.
Regardless of the type of vehicle brake system, the pumps are preceded by hydraulic reservoirs. These reservoirs make pressure fluid available to the pumps and thereby assure pump startup. Known vehicle brake systems use spring piston reservoirs for this purpose. These reservoirs include a piston, guided movably in a reservoir housing, which by circumferential sealing divides a first storage chamber, which can be filled with pressure fluid, from a second storage chamber, which is filled with a gas. The piston is urged by a spring in the direction of its basic position. In this basic position, there is no pressure fluid in the first storage chamber. In known reservoirs, the inflow and outflow of pressure fluid take place through a common supply conduit. Such reservoirs are described for instance in German Patent Application DE 199 42 293 A1.
In the event of an emptied reservoir, to prevent pressure fluid aspirated from the master cylinder by the pump from flowing to the storage chamber, a check valve is disposed in the supply line to such a reservoir. Known check valves have a valve closing body, acted upon by a spring, for controlling a valve seat.
A defective, leaky check valve, in conjunction with the sole supply conduit of the reservoir, would have the effect that the underpressure generated by the self-aspirating pump could affect the wheel brakes. Because of this underpressure, the brake pistons of these wheel brakes will be put in an extreme position. In a subsequent braking event, a disproportionately large amount of pressure fluid would therefore have to be positively displaced into the wheel brakes for the sake of building up brake pressure. The driver could perceive this from a long pedal travel, which could be irritating with regard to the capability of the vehicle brake system to function.
Furthermore, the number of different components for vehicle brake systems with a variable functional scope increases the costs for development and maintenance of a modular system. In addition, relatively many individual components have to be installed to make the known vehicle brake systems, which adversely affects production costs and the resultant structural volume. Known vehicle brake systems furthermore have the potential for improvement in terms of their functional properties when brake slip regulation occurs.